Add oil to a large pot over medium heat. Add garam masala, turmeric, coriander powder and cumin seeds. [Don't include tempering spices which will be added at the end.] Toast spices over medium heat until they are aromatic, about one minute. Add a little water to deglaze.
Add onion, garlic, ginger and chillies, sauté for a few minutes until softened.
Mix in red lentils until well incorporated. Add vegetable broth, cover and let simmer for five to ten minutes.
Separate the can of diced tomatoes in two and puree or crush one half. This step allows you to have half pureed/half chunky. If you don't like chunks, puree the full can or buy tomato already pureed. Add all of the tomatoes to the pot, cover and let simmer for another 15 minutes.